Pollution of rivers and water area such as lakes and marshlands by arsenic is prevailing in Japan and foreign countries, and examples in Bangladesh have been widely reported. Soils in this country are abundant in arsenic, and skin failures and diseases by arsenic are often seen in inhabitants in the country using well water polluted with arsenic as drinking water. While the concentration of arsenic is supposed to reach 0.01 to 1 mg/liter (0.01 mg/liter according to the water quality standard by WhO) in Nawabganj district where the damage is reported to be most serious in Bangladesh, no substantial countermeasures are taken today. On the other hand, pollution by arsenic in Japan is mainly caused by waste water from abandoned mines. The concentration of arsenic in the rivers around the mines ascribed to waste water from tunnels after closing the mines has been reported to be 0.08 to 1.3 mg/liter, which exceeds the environmental standard, around Sasagatani Mine in Shimane Prefecture. In the surrounding area of former Horobetu Sulfur Mine in Hokkaido, the concentration of arsenic in the river had exceeded far above the environmental standard before 1975 due to efflux of strongly acidic water containing a large quantity of arsenic into Benkei River. On the other hand, it was reported that the original arsenic concentration of 0.2 mg/liter had decreased to about 1/10 of 0.02 mg/liter since 1981 thanks to countermeasures to pollution by mining including blocking of entrance of tunnels and installation of a neutralization plant for mine water effected by Sobetsu Town. However, the total amount of use of lime cakes, slaked lime and calcium carbonate used for neutralization and removal of arsenic by coprecipitation in this plant annually accounts for 40,000 to 50,000 tons, and it has been a large problem to ensure landfill sites of a vast amount of precipitates generated from the neutralization plant, and to cover the expense of about 300 million yen per year. There are many water pollution problems by arsenic such as those ascribed to waste water from geothermal power plants and industrial waste other than the problems above. However, since the currently prevailing coprecipitation method involves the problems of waste disposal and treatment cost, treatments by arsenic adsorbents have been expected as substitutes of the arsenic processing method. Particularly, developments of novel adsorbents that can directly adsorb trivalent arsenic are considered to be valuable in establishing an economically advantageous arsenic removal technique, because the process for oxidizing trivalent arsenic into pentavalent arsenic that is relatively easily adsorbed by adding an oxidizing agent may be omitted.
The object of the present invention for solving the conventional technical problems is to provide a novel technique that is able to efficiently remove, to say nothing of pentavalent arsenic, trivalent arsenic that has been considered to be difficult to remove.